Split array into two different arrays using functional JavaScript

Question

I was wondering what would be the best way to split an array into two different arrays using JavaScript, but to keep it in the realms of functional programming.

Let's say that the two arrays should be created depending on some logic. For instance splitting one array should only contain strings with less than four characters and the other the rest.

const arr = ['horse', 'elephant', 'dog', 'crocodile', 'cat'];

I have thought about different methods:

Filter:

const lessThanFour = arr.filter((animal) => {
    return animal.length < 4;
});
const fourAndMore = arr.filter((animal) => {
    return animal.length >= 4;
});

The problem with this for me is that you have to go through your data twice, but it is very readable. Would there be a massive impact doing this twice if you have a rather large array?

Reduce:

const threeFourArr = arr.reduce((animArr, animal) => {
  if (animal.length < 4) {
    return [[...animArr[0], animal], animArr[1]];
  } else {
    return  [animArr[0], [...animArr[1], animal]];
  }
}, [[], []]);

Where the array's 0 index contains the array of less than four and the 1 index contains the array of more than three.

I don't like this too much, because it seems that the data structure is going to give a bit of problems, seeing that it is an array of arrays. I've thought about building an object with the reduce, but I can't imagine that it would be better than the array within an array solution.

I've managed to look at similar questions online as well as Stack Overflow, but many of these break the idea of immutability by using push() or they have very unreadable implementations, which in my opinion breaks the expressiveness of functional programming.

Are there any other ways of doing this? (functional of course)

Answer 1

Although, JavaScript engines are quite efficient and can perform 1000s of loops in a few milliseconds, sorting can also be done using a single loop.

const arr = ['horse', 'elephant', 'dog', 'crocodile', 'cat'];

const lessThanFour = [];
const fourAndMore = [];

arr.forEach(i => (i.length < 4 ? lessThanFour : fourAndMore).push(i));

console.log(lessThanFour);
// Array [ "dog", "cat" ]
console.log(fourAndMore);
// Array [ "horse", "elephant", "crocodile" ]

Answer 2

Kudos for the beautiful response of the user Thank you, here an alternative using a recursion,

const arr = ['horse', 'elephant', 'dog', 'crocodile', 'cat'];


const splitBy = predicate => {
  return x = (input, a, b) => {
    if (input.length > 0) {
      const value = input[0];
      const [z, y] = predicate(value) ? [[...a, value], b] : [a, [...b, value]];
      return x(input.slice(1), z, y);
    } else {
      return [a, b];
    }
  }
}

const  splitAt4 = splitBy(x => x.length < 4);
const [lessThan4, fourAndMore ] = splitAt4(arr, [], []);
console.log(lessThan4, fourAndMore);

Answer 3

collateBy

I just shared a similar answer here

I like this solution better because it abstracts away the collation but allows you to control how items are collated using a higher-order function.

Notice how we don't say anything about animal.length or < 4 or animals[0].push inside collateBy. This procedure has no knowledge of the kind of data you might be collating.

// generic collation procedure
const collateBy = f => g => xs => {
  return xs.reduce((m,x) => {
    let v = f(x)
    return m.set(v, g(m.get(v), x))
  }, new Map())
}

// custom collator
const collateByStrLen4 =
  // collate by length > 4 using array concatenation for like elements
  // note i'm using `[]` as the "seed" value for the empty collation
  collateBy (x=> x.length > 4) ((a=[],b)=> [...a,b])

// sample data
const arr = ['horse','elephant','dog','crocodile','cat']

// get collation
let collation = collateByStrLen4 (arr)

// output specific collation keys
console.log('greater than 4', collation.get(true))
console.log('not greater than 4', collation.get(false))

// output entire collation
console.log('all entries', Array.from(collation.entries()))

Check out that other answer I posted to see other usage varieties. It's a pretty handy procedure.

---

bifilter

This is another solution that captures both out outputs of a filter function, instead of throwing away filtered values like Array.prototype.filter does.

This is basically what your reduce implementation does but it is abstracted into a generic, parameterized procedure. It does not use Array.prototype.push but in the body of a closure, localized mutation is generally accepted as OK.

const bifilter = (f,xs) => {
  return xs.reduce(([T,F], x, i, arr)=> {
    if (f(x, i, arr) === false)
      return [T, [...F,x]]
    else
      return [[...T,x] ,F]
  }, [[],[]])
}

const arr = ['horse','elephant','dog','crocodile','cat']

let [truthy,falsy] = bifilter(x=> x.length > 4, arr)
console.log('greater than 4', truthy)
console.log('not greater than 4', falsy)

Though it might be a little more straightforward, it's not nearly as powerful as collateBy. Either way, pick whichever one you like, adapt it to meet your needs if necessary, and have fun !

---

If this is your own app, go nuts and add it to Array.prototype

// attach to Array.prototype if this is your own app
// do NOT do this if this is part of a lib that others will inherit
Array.prototype.bifilter = function(f) {
  return bifilter(f,this)
}

Answer 4

A shorter .reduce() version would be:

const split = arr.reduce((animArr, animal) => {
  animArr[animal.length < 4 ? 0 : 1].push(animal);
  return animArr
}, [ [], [] ]);

Which might be combined with destructuring:

const [ lessThanFour,  fourAndMore ] = arr.reduce(...)

Answer 5

I don't think there could be another solution than returning an array of arrays or an object containing arrays. How else is a javascript function return multiple arrays after splitting them?

Write a function containing your push logic for readability.

var myArr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
var x = split(myArr, v => (v <= 5));
console.log(x);

function split(array, tester) {
  const result = [
    [],
    []
  ];
  array.forEach((v, i, a) => {
    if (tester(v, i, a)) result[0].push(v);
    else result[1].push(v);
  });
  return result;
}

Answer 6

The function you are trying to build is usually known as partition and can be found under that name in many libraries, such as underscore.js. (As far as I know its not a builtin method)

var threeFourArr = _.partition(animals, function(x){ return x.length < 4 });

I don't like this too much, because it seems that the data structure is going to give a bit of problems, seeing that it is an array of arrays

Well, that is the only way to have a function in Javascript that returns two different values. It looks a bit better if you can use destructuring assignment (an ES6 feature):

var [smalls, bigs] = _.partition(animals, function(x){ return x.length < 4 });

Look at it as returning a pair of arrays instead of returning an array of arrays. "Array of arrays" suggests that you may have a variable number of arrays.

I've managed to look at similar questions online as well as Stack Overflow, but many of these break the idea of immutability by using push() or they have very unreadable implementations, which in my opinion breaks the expressiveness of functional programming.

Mutability is not a problem if you localize it inside a single function. From the outside its just as immutable as before and sometimes using some mutability will be more idiomatic than trying to do everything in a purely functional manner. If I had to code a partition function from scratch I would write something along these lines:

function partition(xs, pred){
   var trues = [];
   var falses = [];
   xs.forEach(function(x){
       if(pred(x)){
           trues.push(x);
       }else{
           falses.push(x);
       }
   });
   return [trues, falses];
}

Answer 7

If you are not opposed to using underscore there is a neat little function called groupBy that does exactly what you are looking for:

const arr = ['horse', 'elephant', 'dog', 'crocodile', 'cat'];

var results = _.groupBy(arr, function(cur) {
    return cur.length > 4;
});

const greaterThanFour = results.true;
const lessThanFour = results.false;

console.log(greaterThanFour); // ["horse", "elephant", "crocodile"]
console.log(lessThanFour); // ["dog", "cat"]